What do you need for a VoIP Network?

As for the PSTN network, to run a VoIP network you need a certain infrastructure, because, even though you use the Internet Network, you still need certain components to use it for voice.
The Internet is the dumb Network that transports packets, but you have to specify which packets and where they are directed.
You usually need:

1) Terminal
A terminal is a communication endpoint that terminates calls and their media streams. Most commonly, this is either a hardware or a software telephone or videophone, possibly enhanced with data capabilities.There are terminals that are intended for user interaction and others that are automated, e.g., answering machines.

2) Server
Terminals usually register their addresses with a server. The server stores
these telephone addresses along with the IP addresses of the respective terminals, and is thus able to map a telephone address to a host.
When a telephone user dials an address, the server tries to resolve the given address into a network address.To do so, the server may interact with other telephony servers or services.
Furthermore a telephony server is responsible for authenticating registrations, authorising calling parties and performing the accounting.(in the case of call termination)

3) Gateway
Gateways are telephony endpoints that facilitate calls between endpoints that usually would not interoperate. Usually this means that a gateway translates one signalling protocol into another.(SIP into ISDN or viceversa or H323 and SIP).
Finding gateways between VoIP and a traditional PBX is usually quite simple. Gateways that translate different VoIP protocols are newer and the future for a really open VoIP.

4) Addressing
Regular telephony systems use E.164 numbers (the international public telecommunication numbering plan). An identifier is composed of up to fifteen digits with a leading plus sign, for example, +1234565789123.When dialling, the leading plus is normally replaced by the international access code, usually double zero (00).This is followed by a country code and a subscriber number.
IP addresses are not location-independent (even if IPv6 is used) and they are hard to remember (especially if IPv6 is used) so they are not suitable as user identifiers.
Current IP Telephony systems use two kinds of identifiers:
- URIs (RFC2396);
- Numbers (E.164).
(you can also use email addresses which are usually in the URL)
The ENUM system was invented, using adapted phone numbers as domain names.

5) Protocols
H 323
SIP (Session Initiation Protocol)
Transport Protocols RTP (Real Time Protocol) and RTCP (Real Time Control Protocol)
Today nearly every vendor that offers VoIP products uses his own VoIP protocol, the enterprise solutions usually feature such proprietary protocols at the cure and provide minimalist support for standardised protocols (until now usually H.323) with only basic call functionality.